The present invention relates to a method of designing transmission power of a synthetic aperture radar capable of greatly reducing the transmission power in the synthetic aperture radar.
In synthetic aperture radars, as known in the art, an antenna having a virtually large aperture is formed using an antenna having rather small aperture. In this technique, an image radar (side-looking radar) is installed on a flight object (platform) such as an artificial satellite or an air craft, and an radio wave is emitted by the image radar, as the flight object is moving, toward the ground in a lateral direction relative to the flight object. The image radar receives reflected radio waves as it moves, and performs synthetic aperture processing on the received radio waves in such a manner that an image equivalent to that obtained via a large-aperture antenna can be obtained. Such a synthetic aperture radar is used as an image sensor that can provide a high-resolution image under all-weather conditions.
FIG. 1 is a schematic diagram illustrating the construction of a typical synthetic aperture radar. In FIG. 1, there are shown a flight object (platform) 11 such as an artificial satellite or an aircraft, a transmitter 12 installed on this flight object, a receiver 13, a duplexer 14, a reception radio wave decoder 15 for recording radio waves received via the receiver 13, and an antenna 16.
Referring to FIG. 2, the operation principle of the synthetic aperture radar constructed with these elements will be described below. The flight object 11 such as an artificial satellite travels at a speed V along an air route or orbit L predetermined depending on a specific purpose. The small-aperture antenna 16 of the synthetic aperture radar installed on the flight object 11 emits transmission radio wave pulses at constant time intervals t.sub.0 at positions A.sub.0, A.sub.1, A.sub.2, . . . along the orbit L at a height h. The transmission radio wave pulse in the form of a beam with a width .beta. is emitted in a direction perpendicular to the orbit L, and it strikes for example an area BCED on the ground at a point A.sub.1. The transmission radio wave pulse is reflected from the ground, travels backward as a reflected wave (radar echo), and is finally received by the same antenna 16.
Reflected waves are received one after another during the flight object 11 moving at the speed V thereby observing a ground area between parallel lines l and l' a distance BC apart and thus recording, in the reception radio wave recorder 15, amplitude information as well as phase information contained in the received signal obtained at each temporal point. If a transmission pulse wave, that was emitted by the flight object 11 when it was for example at the point A.sub.0 , has arrived at a target point P to be detected, then the irradiation of the pulse wave to the target point P starts and the target point P will receive further radiation until it finally receives a pulse wave emitted by the flight object 11 at the point A.sub.2. The radio waves reflected from the target point P during this period are received by the flight object 11. The received radio waves include phase information corresponding to the relative velocity that varies continuously as well as distance information. The received signals are recorded and subjected later to batch processing (holographic processing or synthetic aperture processing) whereby the antenna can act as if it has a great aperture diameter equal to the distance between points A.sub.0 and A.sub.2 (synthetic aperture method).
As described above, signals are received successively at various points and recorded. The received signals are then synthesized such that the antenna can detect a target as if it has a great aperture size a few ten to few ten thousand times the actual aperture size of the antenna. This means that the synthetic aperture radar can have high azimuth resolution and thus can provide a clear image that would be obtained via the equivalent large-aperture antenna.
In conventional designing of transmission power to be output from the transmitter of such a synthetic aperture radar, an radar equation used in ordinary pulse radars is used without any modification. The transmission power is designed such that the reception power satisfies the conditions required for the radar.
The conventionally used radar equation will be described below. Supposing that an radio wave of transmission power P.sub.t is isotropically radiated from the transmit antenna, a power density P.sub.d at a position separated by a distance R may be expressed as: EQU P.sub.d =P.sub.t /(4.pi.R.sup.2) (1)
By setting G.sub.t as gain of the transmit antenna (gain for isotropic antenna), a power density P.sub.d ' in the antenna beam direction may be expressed as: EQU P.sub.d '=P.sub.t .multidot.G.sub.t /(4.pi.R.sup.2) (2)
When an radio wave with such power density is absorbed in a scattering object having an area S, a received power P.sub.sr by the scattering object may be expressed as: EQU P.sub.sr =P.sub.t .multidot.G.sub.t .multidot.S/ (4.pi.R.sup.2)(3)
A proportion of power received by the scattering object is radiated again. Such proportion is referred to as scattering coefficient .sigma..sup.0, and thus the re-radiated power P.sub.st is expressed as: EQU P.sub.st =P.sub.t .multidot.G.sub.t .multidot.S.multidot..sigma..sup.0 /(4.pi.R.sup.2) (4)
Assuming that the power P.sub.st re-radiated from the above scattering object has been isotropically radiated, a power density P.sub.rd to be received at the position of the transmit antenna is expressed as: EQU P.sub.rd =P.sub.t .multidot.G.sub.t .multidot.S.multidot..sigma..sup.0 /(4.pi.R.sup.2) (5)
Here, if the area of the transmit antenna is A.sub.p , a power P.sub.r received by the transmit antenna is expressed as: EQU P.sub.r =P.sub.t .multidot.G.sub.t .multidot.S.multidot..sigma..sup.0 .multidot.A.sub.p /(4.pi.R.sup.2).sup.2 ( 6)
Further, a relation as shown in the following equation (7) exists between the antenna area A.sub.p and the antenna gain G.sub.t : EQU G.sub.t =4.pi.A.sub.p /.lambda..sup.2 (7)
By substituting equation (7) into equation (6), the following equation (8) may be obtained: EQU P.sub.r =P.sub.t .multidot..sigma..sup.0 .multidot.S.multidot.A.sub.p.sup.2 /(4.pi..lambda..sup.2 R.sup.4) (8)
Here, by replacing .lambda..degree..multidot.S with a scattering cross section .sigma. the following equation (9) may be obtained: EQU P.sub.r =P.sub.t .multidot..sigma..multidot.A.sub.p.sup.2 /(4.pi..lambda..sup.2 R.sup.4) where .sigma.=.sigma..sup.0 .multidot.S(9)
This equation (9) constitutes an ordinary radar equation which is conventionally used.
The above radar equation is derived from an assumption, as described, that a scattering object isotropically scatters radio waves. Since, usually, a radar is used to determine an object (mostly discrete object) of which the nature is completely unknown based on scattered radio waves therefrom, the assumption is made that power re-radiated from the object is isotropically radiated. In a sense, an objective determination is made based on such standard for judgment.
In synthetic aperture radars, however, a radio wave is irradiated over a wide range so as to observe a domain extended in a plane. In this case, the scattering of radio waves from a planar object to be observed is no longer isotropic. In theory, thus, the radar equation for an ordinary pulse radar without any modification cannot be used for a synthetic aperture radar. Therefore, in designing the transmission power of a synthetic aperture radar, the transmission power is not correctly designed with the conventional designing method using the unmodified conventional radar equation which is based on the assumption that the power re-radiated from the scattering object is isotropically radiated. The designed transmission power is much larger than what is needed.
With an increased transmission power, the amplification degree of an amplifier must be greater. In addition, it is accompanied by various disadvantages such as a larger calorific value, an occurrence of discharge phenomenon in a transmission line, etc.